This invention relates generally to receivers including signal detect circuits.
When high speed data signals are transmitted over a given media (wire, fiber optics, traces, etc.), the data signals may be degraded by noise and crosstalk picked up along the path between receiver and transmitter. At the receiver end, it is sometimes useful to know whether the input data signal is valid or fatally degraded by noise or crosstalk. As such, data receivers sometimes have a signal detect function to detect whether the input signal is valid.
One way to implement the signal detect function is to look for coding errors in the data stream. If the error rate exceeds a predetermined amount, then the signal is determined to be invalid. This method works well when the coding algorithm can reliably detect very low bit error rates, and the noise corrupting the data causes errors that violate the coding rules for the channel, i.e., the noise is from a source that is not the same as the data being monitored.
One limitation of this method is that it cannot detect crosstalk from an adjacent data channel with the same type of coding. This limitation arises because the interfering crosstalk signal may produce enough noise that meets all the coding requirements of the channel being monitored so that the error detector cannot discern between a valid signal and the crosstalk signal.
Another method used for the signal detect function is to compare the input signal amplitude against a preset threshold. If the input signal amplitude exceeds the preset threshold, then the signal is considered valid. This method is able to reject data transitions due to crosstalk and noise provided the crosstalk and noise amplitude is less than the threshold amplitude.
One limitation of this method is that doing the threshold comparison in the signal path causes duty cycle distortion at the output of the comparator, introducing jitter onto the data signal. A second limitation of this method is that if the interfering noise or crosstalk is greater than the threshold amplitude, this method will indicate that the signal is valid when it actually is invalid. A third limitation with this method is that introducing a threshold comparison in a data path typically degrades the speed of the input comparator, which may pose problems in high speed circuits.
A third method used for signal detect is to do a peak detect on the input signal, integrate it over a long period, and compare it to a preset threshold. One limitation of this method is that if the interfering noise or crosstalk is greater than the threshold amplitude, this method will indicate that the signal is valid when it actually is invalid.
Thus, there is a need for ways of implementing the signal detect function that overcomes one or more problems associated with existing methods.